This disclosure concerns an invention relating generally to methods and apparata for collecting and analyzing gases dissolved within liquids, and more specifically to the collection and analysis of dissolved gas within groundwater.
Groundwater, i.e., subsurface waters such as aquifers, wells, etc. and surface waters such as lakes, streams, etc., is a frequent subject of environmental and biogeochemical study. Often, the characteristics and behavior of the groundwater can be at least partially determined by studying gases dissolved within the groundwater. As examples, dissolved gases are often used to generate estimates of denitrification from excess N2 (1, 2) (these numbers referring to listed documents set forth in a bibliography elsewhere in this document); to study greenhouse gases in groundwater (3,4); to evaluate terminal electron accepting processes using H2 (5); and to study the cycling of biogeochemically important trace gases such as CO2, CH4, and N2O. Additionally, groundwater studies often utilize dissolved gases as xe2x80x9ctracersxe2x80x9d which allow tracking of environmental processes, with applications including paleothermometry (6), age-dating (7, 8), estimating groundwater recharge temperature (9), measuring advection and dispersion in rivers and stream (10), tracing of ocean mixing and circulation paths (11), and tracking of volatile pollutants in groundwater (12), among other applications.
However, the measurement of dissolved gases can be a major analytical challenge. For example, when atmospheric gas concentrations are abundant relative to the dissolved gas concentrations, contamination during sampling and analysis is a major concern. Losses during handling and storage can also be a major problem for some gases due to their volatility and/or biodegradability. In addition, for gases that are typically present at very small concentrations (e.g., SF6 and noble gas isotopes), simply attaining a large enough sample to generate a measurable signal can be a major hurdle.
In view of the foregoing difficulties and the importance of obtaining accurate dissolved gas measurements, a wide variety of sampling and storage procedures have evolved for dissolved gases. Groundwater sampling methods include collection of water in sealed bottles (with or without chemical preservation) for equilibration of gases in the headspace above the water sample (3,4,13); flame sealing of water samples in glass ampules under high purity gases to protect the sample from contamination with the atmosphere (8); collection in copper tubes with stainless steel pinch-offs (14); bubble stripping, diffusion probes, and downhole samplers (15); and others. Complex devices and methods may also be required to extract the gases from the groundwater and to introduce and process gas samples within an analytical instrument. These include, for example, purge and trap devices for analysis of VOCs, trace gases, CECs and SF6 (7 and 8), and highly refined vacuum extraction devices in-line with analytical instruments (14).
The foregoing devices and methods for groundwater sampling and dissolved gas collection suffer from the problems that they can be time-consuming, expensive, and difficult to operate (particularly in field conditions). Thus, it would be useful to have available further devices and methods for groundwater sampling and dissolved gas collection and analysis which at least partially overcome some of these difficulties.
The invention involves methods and apparata for dissolved gas collection which are intended to at least partially solve the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions of the collection devices and methods. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
An apparatus for collecting dissolved gases from groundwater is provided wherein the groundwater of interest for study is supplied to an intake line having an inlet end and a downstream outlet end. One or more restrictor lines are then provided downstream from the outlet end of the intake line, with the restrictor lines promoting a hydrostatic pressure drop in the groundwater traveling therein. This pressure drop can be generated owing to pumping forces working against resistance (i.e., frictional losses) from the restrictor lines, which resistance may be enhanced by decreasing the flow area of each restrictor line (i.e., the area of the flow passage measured perpendicular to the direction of flow) and/or increasing the length of each restrictor line. Alternatively or additionally, a pressure drop can be generated or enhanced by having the combined flow areas of the restrictor lines be less than the flow area of the intake line, whereby the fluid velocity of the groundwater is increased to generate a corresponding pressure drop. When the groundwater is supplied from the intake line and its pressure is decreased within the restrictor lines, the dissolved gases (if any) within the groundwater will precipitate from the groundwater. Thus, the dissolved gases begin to bubble from the groundwater flowing within the restrictor lines.
A gas collection chamber is then situated downstream from the restrictor line(s), with the gas collection chamber having an interior wherein the precipitated gas bubbles may collect near its upper side. Thus, as the flow of groundwater continues, the amount of collected gas at the upper side of the gas collection chamber grows. A disposal line then exits the gas collection chamber, preferably near the lower side of its interior and below the level at which the groundwater from the restrictor lines enters the gas collection chamber, so that degassed groundwater will flow from the disposal line but the collected gases remain within the gas collection chamber.
If the source of the groundwater is not pressurized such that it will flow through the lines of the apparatus of its own accord (as where the groundwater is supplied from an artesian well), a pump may be situated somewhere among or between the foregoing components to induce flow. A preferred arrangement is to situate any pump downstream from the restrictor lines so that the pressure drop within the restrictor lines (and thus the precipitation of any gases from the groundwater within the restrictor lines) is enhanced by the pump suction.
To remove at least a portion of the collected gases from the gas collection chamber, a sampling port is preferably provided at or near the upper side of the interior of the gas collection chamber where the precipitated gases collect. The sampling port preferably bears a valve, and is adapted for removable attachment of a sample collector (such as a syringe) having an adjustable interior volume. Thus, the valve may be opened and the collected gases may be drawn off into the sample collector to be provided to a suitable analysis device. The collected gases may then be analyzed by gas chromatography or other methods to determine their contents. Alternatively, analytical instrumentation for gas analysis can be directly connected to the sampling port so that analysis can occur concurrently with collection of gases.
Conveniently, the foregoing apparatus may be provided in a portable and easily disassembled and reassembled form so that it may be easily used in the field as well as in a laboratory. As an example, the intake and restrictor lines may be provided by easily folded flexible plastic tubing, and the collection chamber (which is preferably formed of rigid plastic or glass) can be sized so that it may be easily held by one hand. The pump (if present) may take the form of a laboratory peristaltic (or other) pump which has a protruding input shaft adapted for receiving a rotary input from a cordless drill or other easily portable source of a rotary power input. Some of all of the foregoing components may be transparent, allowing the color and particulate content of the sampled groundwater to be observed during gas collection, and allowing any areas of actual or potential fouling to be observed during gas collection.
The invention offers several advantages for gas collection and analysis applications. Initially, it allows multiple sample collection procedures to be combined into one simple method. To illustrate, traditional sampling techniques might require
(a) that water samples to be analyzed for CFCs be flame-sealed in glass ampules under high purity N2, with subsequent analysis by purge and cryogenic trap (8),
(b) that water samples to be analyzed for SF6 be collected in a 1-liter or larger bottle with a special cap, with subsequent analysis by purge and cryogenic trap (7),
(c) that water samples to be analyzed for Ar, N2, and O2 (which is unstable during storage) be collected in a 50-ml septum-sealed glass bottle, with subsequent analysis by a headspace method (13),
(d) that water samples to be analyzed for H2 be run through a bubble gas stripping chamber to collect H2, with subsequent analysis by direct injection (15), and
(e) that water to be analyzed for CH4 and N2O be injected into a He or Ar flushed septum-sealed glass bottle, with subsequent analysis of the CH4 and N2O in the He or Ar headspace (13,3).
In contrast, by use of the invention, one only need collect one or more samples of the collected gas in the field or elsewhere, using a syringe or other sample collector, for later direct injection into a gas chromatograph in the laboratory (though purge and trap and/or other methods may be used if desired). Alternatively, the collected gases can be continuously supplied to analytical instrumentation concurrently with their collection.
Further, the invention avoids the consumption and production of biogenic gases that may occur during storage of a water sample for later analysis. Biotransformation of gases such as CO2, O2, CH4 and N2O during storage of water samples causes over- or underestimation of the in situ concentration of such gases. Since the invention separates the gases from the water when the water is sampled, only the gas need be stored, thus avoiding aqueous bio-transformations during storage.
Additionally, the invention allows the collected gases to be directly supplied to or injected into analytical instrumentation. More elaborate sample introduction and processing approaches (e.g., the valving required to purge and trap CFCs and SF6 from water samples) may be unnecessary depending on the application in question.